The present invention relates to a method and a device for recognizing cornering, especially over-steered cornering, and for stabilizing a vehicle in case of over-steered cornering.
A cornering maneuver can be recognized by different sensors, for example, steering angle sensors or transverse acceleration sensors, but the additional expenditure for the sensors also increases the expenditure for the cabling, the costs and the failure probability. Thus, there are applications in which it is desirable that cornering be detected without additional sensors. By the way, it is often difficult to recognize over-steered cornering, which is understood as a cornering maneuver in which the vehicle turns into a curve around its vertical axis to an extent exceeding the extent that would be necessary or, more generally speaking, in which a vehicle drives to the outer side of the curve with its tail. In the extreme case, we are talking about a swerving car in the broadest sense of the word. The present invention considers in particular also the extreme cases in which an over-steering exists only to a relatively small extent, for example, at the beginning of the vehicle""s swerving. It is difficult to recognize an over-steered cornering maneuver just in these cases so that the over-steered behavior is increasing slowly until finally the vehicle is completely unstable. Conventional methods for recognizing over-steered cornering are not very useful due to the limited transverse dynamics in the limit range, so that the response thresholds for stabilizing interventions are not reached. Thus, a stabilizing brake intervention, which in principle would be possible, is omitted due to the lacking or delayed recognition of the over-steered cornering maneuver.
It is the object of the present invention to provide methods and devices for recognizing cornering, especially over-steered cornering, as well as for stabilizing a vehicle during an unstable cornering maneuver, which are sensitive, reliable and manage without additional expenditure for sensors, if necessary.
Before describing single embodiments of the invention, basic relations of a vehicle in which the present invention can be applied are illustrated with regard to FIG. 1 and FIG. 2. FIG. 1 schematically shows a vehicle. Reference numerals 101 to 104 are the wheels of the vehicle, reference numeral 101 being the left front wheel, reference numeral 102 the right front wheel, reference numeral 103 the right rear wheel and reference numeral 104 the left rear wheel. Reference numeral 105 is the front axle, reference numeral 106 the rear axle. Reference numerals 111 to 114 are the wheel sensors detecting the wheel speed of the single wheels, particularly the rotating speed. Reference numerals 121 to 124 symbolize the wheel brakes. The output signals of the wheel sensors 111 to 114 are transmitted to a control 130. Furthermore, the control can also receive signals of additional sensors 115 to 117. In addition, the control 130 produces output signals 131 with which the longitudinal dynamics and/or the transverse dynamics of the vehicle can be influenced. Thus, they produce in particular signals for the wheel brakes 121 to 124 in order to adjust the brake pressure. In addition, signals can be produced which influence the driving torque and, if necessary, also the automatic transmission.
If a vehicle drives around a comer transverse forces (with regard to the longitudinal axis of the vehicle) have to be produced counteracting on the one hand to the centrifugal force resulting from cornering, and on the other hand to the moment of inertia of the vehicle itself during steering. The wheels transmit these forces to the vehicle. If the vehicle is stable the transverse forces resulting from this process can be transmitted by means of the static friction between roadway and tire. If the vehicle is unstable and in particular if it is over-steered, the transverse forces that are actually necessary are bigger than the forces that can be transmitted due to the static friction between roadway and wheels.
FIG. 2 describes the case that might appear in an over-steered left-hand curve. In FIG. 2, the left front wheel is shown. The same reference numerals as in FIG. 1 indicate the same components. Reference numeral 111 is the wheel sensor, reference numeral 111a is a marking disc which follows the wheel 101 and helps determining the rotating speed of the wheel 101. The speed of the wheel (and vehicle as described below) on the roadway is marked Vf. It is not oriented parallel to the wheel plane (vertical to the wheel axis) but extends at an angle, xcex1, to the wheel plane. FIG. 2 shows the case of a wheel that is not braked. In this case, it can be assumed that the speed of the wheel in the wheel plane (vertical to the wheel axis) corresponds to the speed component of the wheel on the roadway (because the wheel can freely roll). As a result, the vehicle speed, Vf, can be determined from the vectorial addition of the longitudinal component, Vl, and the transverse component, Vq. More specifically, if there is a difference between the vehicle speed Vf and the longitudinal component, Vl (detected by the wheel sensors), then the difference can be attributed to a transverse component, Vq. This is valid both for the vectorial approach as also for the approach by the absolute amount.
Furthermore, it was established that during each cornering (i.e. ultimately also if a cornering maneuver is considered as stable) there is a transverse componentxe2x80x94even if it is smallxe2x80x94so that during each cornering, whether stable or over-steered, a transverse component is produced. Thus, a speed difference between the vehicle speed, Vf, and the longitudinal component, Vl (slip). The slip (difference between vehicle speed, Vf, and longitudinal component, Vl, or the difference between their absolute amounts) can be produced on different wheels, according to the driving situation.
According to the present invention, cornering is determined with reference to several slip values on several vehicle wheels. Also, an over-steered cornering maneuver can be determined with reference to several slip values on several vehicle wheels. According to the present invention, over-steered cornering can also be determined with reference to the transverse accelerations of the vehicle axles. According to another aspect of the present invention, over-steered cornering can be determined in a particularly reliable way if the determination based on the wheel slip values and the determination based on the transverse accelerations of the axles are combined with each other. If over-steered cornering has been detected, according to the present invention one or more measures supporting stability can be taken.
In the following single embodiments of the invention are described on the basis of the figures, whereby: